Optical position sensors are used to monitor the location of an optical spot that is incident upon the active area surface of a device. They are commercially available in a one-dimensional and a two-dimensional version, which are also commonly called a Position Sensitive Detector (PSD) or a Position Sensitive Photo Detector (PSPD).
Two types of monolithic position sensitive photo detectors are widely available for measurements in one dimension. The lateral effect detector incorporates an electric resistive layer over the active surface area of a single photo diode, with electrical contacts at either end of the layer. This type of detector is useful for measuring the centroid of an optical spot that may move across the entire photosensitive area. A second type, called the bi-cell, is sensitive to displacements that are small compared to the size of the optical spot, and commonly is used to monitor perturbations of a probe beam caused by mechanical vibrations or optical misalignment. The bi-cell, as the name already indicates, is composed out of two cells, I, 1 and, II, 2, that are adjacent to each other and have a small gap 3 between them, See FIG. 1A. The surface of each cell represents a photodiode. The photodiodes are fabricated on a monolithic chip and share the cathode 4 and have an individual anode, 5 and 6, see FIG. 1B. In a position detector system, see FIG. 1C, their anodes are typically connected to transimpedance amplifiers 7 or so called I-V converters that convert the current signals from the photodiodes, II and III that run through wire 5 and 6, into inverted voltage signals −UI and −UII at 10 and 11. A differential/sum amplifier 12 subtracts −UI from −UII and outputs the result (UI−UII) on 13. A summing amplifier 14 sums −UI and −UII, inverts this, and outputs the result (UI+UII) on 15. The I-V converters, the differential/sum amplifier and the summing amplifier are typically constructed using operational amplifiers (opamps).
The difference signal (UI−UII), 13, is connected to the divider 16 at the signal input 17 pin. The sum signal (UI+UII), 15, is connected to the denominator input, 18, of the divider. The output of the divider, 19, represents the one-dimensional position of the optical spot normalized with respect to the detection range and is mathematically represented by PID=(UI−UII)/(UI+UII).
Theoretically the difference signal (UI−UII) corresponds with the location of the optical spot on the active areas, and the sum signal (UI+UII) corresponds with the total light intensity on the active areas. In order to get a position value PID that depends on the light spot size and is independent of the total light intensity the normalization is performed, which is done by dividing the difference signal with the sum signal. This links the output voltage directly to the size of the optical spot making it sensitive to small position perturbations of the beam and reduces noise that is induced by light intensity variations.
The two-dimensional monolithic PSDs are also widely available in two types, which are derived from the one-dimensional version. There is a duo lateral effect detector (L. Lindholm, PCT/SE2001/001235) which is useful for measuring the two-dimensional position of the spot centroid across the entire photosensitive area. And there is a quad-cell, or so called quadrant photodiode or four-segment photodiode that is sensitive to displacements that are small compared to the size of the optical spot and is frequently used in Atomic Force Microscopy (AFM), Friction Force Microscopy (FFM), Scanning Probe Data Storage, optical levers, autocollimators, optical beam profiling, laser beam position sensing, and other alignment applications. Like the name indicates, it is composed out of four active cells, denoted A, B, C, and D, which are ordered in a 2×2 array and are separated by a small gap, see FIG. 1D. Like the bi-cell, each active cell of the quad-cell represents a photodiode which are fabricated together on a monolithic chip and have a shared cathode, 21, and a individual anode, 22, 23, 24, 25, see FIG. 1E. Atypical two-dimensional position detector system is constructed using opamps and contains four I-V converters, 26, two sum/difference amplifiers, 27, one summing amplifier, 28, and two dividers, 29, See FIG. 1F. There are two electrical outputs, 30 and 31, one for the normalized vertical position signal, PV=(UA+UB−(UC+UD))/(UA+UB+UC+UD), and one for the normalized horizontal position signal, PH=(UB+UD−(UA+UC))/(UA+UB+UC+UD).
Like with the bi-cell, the normalization is performed by dividing the difference signal with the sum signal in order to get the normalized vertical position value PV respectively a normalized horizontal position value PH. The two difference signals, (UA+UB)−(UC+UD) and (UB+UD)−(UA+UC), are analogs of the relative intensity difference of the light sensed by opposing pairs of the photodiode quadrant elements. The sum signal, (UA+UB+UC+UD), is the analog of the intensity sensed by all four quadrant elements together.
The conventional bi- or quad-sensors require I-V and sum/subtraction stages in order to retrieve the position signals from the photo sensitive segments. The I-V stage transforms the current coming out of the photo segments into a corresponding voltage which then by means of the sum and difference amplifiers is transformed into a voltage type position signal. This means that the signal traverses two sequential opamp stages before the position signal can be normalized, see FIGS. 1C, and 1F.
The use of opamps has serious downsides. Wideband I-V converters tend to be unstable (pag. 29, Mark Johnson. Photodetection and Measurement, Mc Graw-Hill, ISBN 0-07-140944-0) and add unwanted oscillatory signals to the position signal that decreases the maximum obtainable position resolution. Furthermore, conventional frequency-compensated opamps use an internal RC combination to give a dominant frequency pole at around 20 Hz. Above this frequency the gain drops off at a rate of −20 dB/decade, reaching 0 dB (unity gain) at the frequency corresponding to the Gain Bandwidth Product (GBW). The gain is therefore an approximately inverse function of frequency over the useful frequency range. The gain at any frequency f is approximately GBW/f, and the upper frequency limit or bandwidth of the transimpedance amplifier is: flimit=(GBW/{flimit2πRlCp})1/2. This is an approximate expression (pag. 29, Mark Johnson. Photodetection and Measurement, Mc Graw-Hill, ISBN 0-07-140944-0) and should not be relied on for exacting accuracy. Typically the limiting frequency is half the value calculated from this expression. So, due to the use of opamps the bandwidth of the position sensor is severely limited.
For a typical wideband photodiode transimpedance amplifier consisting out of a 12 pF photodiode connected to an OPA657 opamp having a GBW of 1.6 GHz and a 200 kOhm feedback resistor, the bandwidth is 10 MHz (Texas Instruments, OPA 657 Datasheet).
Also, the sum and difference amplifiers have a bandwidth that is limited by the Gain Bandwidth Product (GBP) of the opamps used. (P. Horowitz, The art of electronics, Cambridge University Press). The sum and sum/subtraction stage also contains a feedback resistor, see FIG. 1.F, and when concatenated to the above wideband photodiode transimpedance amplifier would reduce the bandwidth of the sensor system even further, to values below 10 MHz.
The fastest optical position sensor reported is the sensor used by Toshio Ando of Kanazawa University Japan (T. Ando et. al., Eur J Physiol, DOI 10.1007/s00424-007-0406-0, Springer-Verlag, 2007) and is used in an optical beam deflection detector for high speed atomic force microscopy and nano visualization of biomolecular processes. The sensor is based on a four-segmented 3 pF 40 MHz Si Pin photodiode and a custom-made fast amplifier/signal conditioner having a bandwidth of about 20 MHz. Usually in high speed microscopy the position signals are not normalized because analog signal dividers are limited to about 10 MHz (Analog Devices, AD734) and real-time digital division introduces unwanted signal delays.
Furthermore, the feedback resistors of the I-V stage and the sum/subtraction stage are a source of thermal noise known as Johnson noise. It is highly desirable to be able to measure the position of an optical spot with the least electrical noise possible (J. D. Spear, low noise optical position sensor, pat. US005880461A). Spear has invented a separate-bi-cell photodetector that does not require the difference amplifier and therefore reduces the Johnson noise of the system by the elimination of that stage. His design however still requires the use of the bandwidth limiting opamp I-V amplifier or so called current-to-voltage amplifier.
In his design, two separate photo segments are connected in parallel to the I-V converter doubling the capacitance on the input of the I-V converter with respect to that of a single photodiode connected. Because the bandwidth of the sum/difference amplifier is much larger than that of the I-V converter the connection of the second photodiode to the opamp about halves the bandwidth of the position sensor system. Also, because still the opamp I-V converter is used the unwanted oscillatory distortion is still present in the output signal in wideband applications together with the Johnson noise that originates from the feedback resistor.
Furthermore the distortion of the position signal due to light intensity fluctuations cannot be normalized out of this system because the sum signal cannot be distracted from it. Their design demands that the two photodiodes have to be connected in parallel to each other, “with the cathode of the one photodiode connected to the anode of the other photodiode and the anode of the said one photodiode connected to the cathode of the said other photodiode”. This parallel connection leaves no connection left for the independent determination of the currents running through each photodiode so that that the sum signal cannot be retrieved. Hence making it impossible for the position signal to be normalized.
So, for wideband high sensitivity position measurements of an optical beam the conventional detector cells having the standard two stage opamp circuitry in combination with an analog signal divider are considered to be the best measuring device available.
In our inventive two-dimensional optical position sensor the decomposition of the two-dimensional input signal into two times a one-dimensional signal is performed optically so that the electrical determination of the position and sum signals can be performed for each dimension independently from the other dimension. This enormously increases bandwidth and signal-to-noise ratio, and decreases cross couplings, noise, and system complexity.
Our inventive light beam position sensor does not require I-V amplifiers nor does it require additional sum and/or sum/subtraction stages for the retrieval of one-dimensional or two-dimensional position signals. Hence it is not limited in bandwidth due to these amplifiers and interconnections, and does it not contain the thermal noise and oscillatory distortions these elements produce.
Also does our inventive sensor permit the retrieval of signals from the photo-segments for the construction of the sum signal. We have also invented a wideband method to perform the summing required without interfering with the position signals. Hence, a wideband means for the sum signal is provided while keeping the position signals wideband also.
Additionally, to solve the normalization problem, we have invented a wideband normalization method capable of normalizing signals up to the conversion rate of the fastest analog-to-digital converters available, which is nowadays about 2 GS/s for a single analog-to-digital converter. Interleaved analog-to-digital converter systems would increase the bandwidth of the normalization method even further. This normalization method does not require any extra components at all when the position signal is to be digitized in the application where it is used in. This would result in an enormous system simplification, cost reduction, and energy saving, as the outputs of conventional position sensors are almost always digitized for recording or processing purposes.
With the conventional sensor there is a separation gap, 3, between the active areas. In cases where a non-homogenous light spot is used errors occur due to this gap. This error becomes larger when smaller photodiode areas are used because the area width to gap width ratio decreases. The gap makes it impossible to use small photodiode areas which are faster in response due to their lower internal capacitance.
In our inventive sensor there is no separation gap necessary between the segments so that any errors that are produced due to this separation gap are not present. This opens the opportunity to focus the optical beam on small photodiode areas without the gap error.
In the prior art sensors back reflection of light in the direction of the incoming beam can result in problematic errors. This can be prevented in our inventive detector. It reduces optical pollution—which is about 40% of the incident light—in the application wherein the sensor is used. In applications where coherent light is used pollution due to interference effects of the incoming bundle with the back scattered bundle are thus removable.
In the alignment of a conventional two-dimensional sensor the sensor needs to be moved as a whole in the plane of the position-detection with the result that by movement of the center position of the first axis there is always some uncontrollable movement of the center position of the second axis. This unwanted dis-alignment of the second axis center then requires supplemental alignment which in turn dis-aligns the first axis center again. So, the alignment of the conventional sensor is an iterative process requiring multiple steps which is time-consuming to do manually and is difficult to make automatic due to the iteration process involved.
Our inventive sensor has an unexpected result. It permits the horizontal and vertical center positions to be adjusted without cross-couplings to each other so that the alignment of the axis can be done in one step which can be quickly performed manually and would be easily to implement automatically.
Thus a need exists for a one-dimensional and two-dimensional optical position sensor which allows high bandwidth without sacrificing signal to noise ratio unnecessary. There is further need for a high speed optical position sensor that also outputs the total intensity (sum) signal at high bandwidth. There is also a need for an optical position sensor which allows electrical design flexibility in its output properties for integration with other electronics. There is also a need for the high and wide bandwidth normalization of a position signal with respect to a sum (total intensity) signal.
Furthermore there is a need for an optical position sensor that; has a low amount of back scattered light, that allows easy and direct alignment without cross coupling among its axis, that has a low temperature drift, and has no dead gap between the photo sensitive segments.